Dipole matrix elements and the nature of charge oscillation under coherentinterband excitation in quantum wells

An empirical pseudopotential method is used to model two type-I quantum-wel l systems, allowing the investigation of interband dipole-matrix elements a nd charge oscillation under coherent optical excitation. Each relevant (mic roscopically varying) wave function is expressed as an exact envelope-funct ion expansion to which various approximations are made, in analogy with env elope-function methods such as the k.p model. The approximation to the quan tum-well energy eigenfunctions of a single envelope function multiplying a band-edge zone-center state, the "atomic picture,'' is shown to underestima te by orders of magnitude the interband dipole-matrix element. Including te rms due to the second band edge, which play only a minor role in the exact envelope-function expansion, provides a good approximation to the true dipo le-matrix element, which is significantly greater than the atomic picture p redicts. In addition, the effect on the interband charge oscillation of omi tting the second band-edge terms is shown to be a reduction of the oscillat ion from the width of the well to the atomic scale. These results confirm t hat the earlier results of Burt hold for realistic three-dimensional system s. [S0163-1829(99)03343-3].